Struct rustc::traits::ProjectionCache

pub struct ProjectionCache<'tcx> {
    map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
}

🔬 This is a nightly-only experimental API. (rustc_private)

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The projection cache. Unlike the standard caches, this can include infcx-dependent type variables - therefore, we have to roll the cache back each time we roll a snapshot back, to avoid assumptions on yet-unresolved inference variables. Types with skolemized regions also have to be removed when the respective snapshot ends.

Because of that, projection cache entries can be "stranded" and left inaccessible when type variables inside the key are resolved. We make no attempt to recover or remove "stranded" entries, but rather let them be (for the lifetime of the infcx).

Entries in the projection cache might contain inference variables that will be resolved by obligations on the projection cache entry - e.g. when a type parameter in the associated type is constrained through an "RFC 447" projection on the impl.

When working with a fulfillment context, the derived obligations of each projection cache entry will be registered on the fulfillcx, so any users that can wait for a fulfillcx fixed point need not care about this. However, users that don't wait for a fixed point (e.g. trait evaluation) have to resolve the obligations themselves to make sure the projected result is ok and avoid issues like #43132.

If that is done, after evaluation the obligations, it is a good idea to call ProjectionCache::complete to make sure the obligations won't be re-evaluated and avoid an exponential worst-case.

FIXME: we probably also want some sort of cross-infcx cache here to reduce the amount of duplication. Let's see what we get with the Chalk reforms.

Fields

map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>

🔬 This is a nightly-only experimental API. (rustc_private)

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Methods

impl<'tcx> ProjectionCache<'tcx>

pub fn new() -> Self

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pub fn clear(&mut self)

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pub fn snapshot(&mut self) -> ProjectionCacheSnapshot

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pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot)

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pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot)

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pub fn commit(&mut self, snapshot: &ProjectionCacheSnapshot)

🔬 This is a nightly-only experimental API. (rustc_private)

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fn try_start(     &mut self,     key: ProjectionCacheKey<'tcx> ) -> Result<(), ProjectionCacheEntry<'tcx>>

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Try to start normalize key; returns an error if normalization already occurred (this error corresponds to a cache hit, so it's actually a good thing).

fn insert_ty(     &mut self,     key: ProjectionCacheKey<'tcx>,     value: Normalized<'tcx, Ty<'tcx>> )

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Indicates that key was normalized to value.

pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>)

🔬 This is a nightly-only experimental API. (rustc_private)

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Mark the relevant projection cache key as having its derived obligations complete, so they won't have to be re-computed (this is OK to do in a snapshot - if the snapshot is rolled back, the obligations will be marked as incomplete again).

pub fn complete_normalized(     &mut self,     key: ProjectionCacheKey<'tcx>,     ty: &Normalized<'tcx, Ty<'tcx>> )

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A specialized version of complete for when the key's value is known to be a NormalizedTy.

fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>)

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Indicates that trying to normalize key resulted in ambiguity. No point in trying it again then until we gain more type information (in which case, the "fully resolved" key will be different).

fn error(&mut self, key: ProjectionCacheKey<'tcx>)

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Indicates that trying to normalize key resulted in error.